1. Field of the Invention
The present invention relates to a turbocharger which has a rotary electric machine mounted on the rotatable shaft thereof and operable as a motor-generator which functions as a motor or a generator.
2. Prior Art
There have heretofore been widely used turbochargers which have a turbine rotatable by the energy of exhaust gases emitted from an internal combustion engine and a compressor mounted on the rotatable shaft of the turbine, for compressing intake air and supplying compressed intake air to the engine. Various proposals have been made in which a rotary electric machine operable as a motor-generator is mounted on the rotatable shaft of the turbocharger and is operated as a motor or a generator depending on operating conditions of the engine.
Japanese Laid-Open Patent Publication No. 62(1987)-48931 disclosed a turbocharger for an internal combustion engine. In the disclosed system, when the engine operates in a low-speed, high-load range, electric power is supplied to the rotary electric machine to operate the same as a motor, thereby assisting the turbocharger in supercharging the engine and when the engine rotates at high speed, the rotary electric machine is operated as a generator.
The conventional turbocharger is greatly affected by changes in exhaust gases emitted from the engine. More specifically, when the engine operates at low speed and under partial load, the exhaust gases are discharged from the engine under low pressure and at low rate. Particularly, while the engine is idling, the energy of discharged exhaust gases is so small that it is almost impossible to rotate the turbine of the turbocharger, and hence no electric power is generated by the rotary electric machine.
If a turbine designed to operate when exhaust gases are emitted under low pressure and at low rate is employed, when exhaust gases are emitted under high pressure and at high rate, the turbine would be excessively rotated and damaged. To avoid this shortcoming, exhaust gases have to be discharged through a bypass around the turbine, in which case the energy of exhaust gases cannot fully be utilized effectively.
There has been proposed another arrangement which employs two scrolls for introducing exhaust gases into a turbine and two nozzles directed from the scrolls toward turbine blades. When the engine operates at low speed and under partial load, one of the nozzles is closed to increase the speed of flow of exhaust gases through the other nozzle. While the above problems are somewhat solved by this system, the performance of the turbine cannot fully be achieved since the exhaust gases are applied to the turbine blades at different positions depending on the amount of applied exhaust gases.